Biogas from agricultural waste streams represents an important way to produce fossil-free energy, allow nutrient recycling and reduce greenhouse gas emissions. However, biogas production from agricultural substrates is currently far from reaching its full potential In Sweden, the number of biogas plants and their output have increased in recent years, but they are still experiencing harsh economic conditions. A recent evaluation (2010-2015) of 31 farm-scale biogas production facilities in Sweden sought to identify parameters of importance for further positive development. In this paper, data on plant operation, gas yield and digestate quality for 27 of these plants are summarised and statistically analysed to investigate factors that could allow an increase in overall biogas production and in nutrient content in the digestate. The analysis showed that addition of co-substrates to manure results in higher gas production, expressed as both specific methane potential and volumetric gas production, than when manure is the sole substrate. Use of co-substrate was also found to be influential for the nutrient content of the digestate. These observed improvements caused by co-digestion should be considered when subsidy systems for manure based biogas processes are being created, as they could also improve the economics of biogas production. However, to achieve higher efficiency in existing biogas plants and to improve the situation for future investments, a more detailed, long-term evaluation programme should also be considered. (C) 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license.